1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to a fusing apparatus having current blocking means blocking current applied to a heating roller to substantially prevent overheating of the heating roller, and an image forming apparatus using the fusing apparatus.
2. Description of Related Art
Generally, a color image forming apparatus using an electrophotography scheme is an apparatus for forming a color image by irradiating light on a photosensitive medium charged with a predetermined electric potential to form an electrostatic latent image corresponding to an image. The electrostatic latent image is developed with predetermined color toners. The developed image is transferred and fused on a print medium.
FIG. 1 is a longitudinal cross sectional view schematically showing a construction of a conventional fusing apparatus.
Referring to FIG. 1, a fusing apparatus 10 includes a fusing unit 11 and a pressing roller 15 facing the fusing unit 11.
The fusing unit 11 is a unit applying heat to a toner image transferred on a print medium 17 and includes a pipe-shaped cylindrical fusing roller 12 and a heat generating unit 14 generating heat by using current transmitted from an external power supply (not shown) provided to an inner side of the fusing roller 12.
A release layer 13, which is preferably made of an elastic material to improve releasability to the toner image, is provided on a circumferential surface of the fusing roller 12.
Radiation energy (heat) generated by the heat generating unit 14 is transmitted to the fusing roller 12 through air charged in an inner portion of the fusing roller 12 and converted to thermal energy by an opto-thermal conversion layer (not shown) coated on an inner side surface of the fusing roller 12 to heat the fusing roller 12, and then the release layer 13 is heated up to a predetermined fusing temperature due to thermal conduction.
A pressing roller 15 facing the fusing unit 10 through a print medium 17 is disposed under the fusing unit 11. The pressing roller 15 is elastically supported by a spring member 16 to press the print medium 17 passing between the fusing unit 10 and the pressing roller 15 toward the fusing unit 11.
At this time, the powder-state toner image 18 transferred on the print medium 17 is pressed and heated with predetermined pressure and heat while the print medium 17 passes between the fusing unit 11 and the pressing roller 15. The toner image 18 is fused on the print medium 17 with predetermined temperature heat and pressure by the fusing unit 11 and the pressing roller 15.
A thermostat 20 is disposed over the fusing unit 11 for stopping power supply to prevent overheating of the surface temperature of the release layer 13 when the surface temperature rapidly increases. A thermistor 30 is disposed over the fusing unit 11 for measuring the surface temperatures of the fusing roller 12 and the release layer 13.
However, the thermostat 20 is located at a position separated from the fusing unit 11 by a distance of about 1 mm to 2 mm, and a temperature of a surface of the release layer 13 is not directly sensed. Instead, the thermostat 20 measures a temperature of air heated around the release layer 13.
Therefore, due to the separation of the thermostat 20 and the fusing unit 11, response capability is deteriorated, and the power supply cannot be instantaneously blocked when an abnormal state occurs due to actual overheating. Thus, a problem exists that an emergency, such as a fire, may occur.
Therefore, a need exists for an image forming apparatus having an improved fusing unit adapted to substantially block current transfer when a temperature of the fusing roller rapidly increases.